Vehicles of more recent design have drive units whose efficiency is very much improved as compared with earlier engines. This means that less waste heat is available to heat such vehicles; in other words, if additional actions are not taken, their occupants freeze when the weather is cold. This is particularly the case for diesel vehicles, but also for vehicles having fuel cell drive systems, for light aircraft, and for gasoline-powered vehicles with a hybrid drive system.
Additional heating units, which are operated in some cases with fuel, and in other cases with electricity, are therefore used. For example, by means of a small electric heating unit, heated air can be blown into the driver's seat and likewise into the rear footwell in order to effect heating there. By means of such heating units, warm air can be produced at the diesel filter so that the heat prevents paraffin from precipitating there.
In order to enhance traffic safety and occupant comfort, however, heating units of this kind should be generally available in order to heat not only the driver's seat, the rear footwell, and the diesel filter, but the entire passenger compartment, the steering wheel, windshield, etc. A resulting essential requirement for such heating units is therefore that they must work effectively already in the first few minutes after the vehicle is started, so that the temperature rises quickly.